Research led by Swansea University academics shows that European summer temperatures can be linked to random changes in the position of the storm track crossing the Atlantic.

Whilst average European temperatures, on the whole, rise and fall in response to climatic forcing factors, such as greenhouse gases, there is another factor that impacts European summer climate strongly - the position of the Atlantic storm track, which varies randomly and could lead to a mixture of cold and wet or warm and dry summers in the future, that we simply cannot predict.

Researchers from the Geography Department at the College of Science published the study in Nature Geoscience. It shows that that although the average summer temperature of Europe has warmed and cooled in the past, in response to external factors such as greenhouse gases, volcanic eruptions and the amount of energy coming from the sun, the variation in temperature between northern and southern Europe has changed in an unpredictable way not linked to these external climate drivers. This unpredictability could mean a steady rise in summer temperatures that is interspersed with successions of unusually warm and dry, or cold and wet summers in the future.

Lead author, Dr Mary Gagen of Swansea University, said: “This is not good news. People do not directly feel the impact of the annual temperature of the globe, or even of a continent; people’s lives are impacted by the climate of a particular season, or year, in the place where they live. In terms of quality of life, there is a big difference between a pleasantly warm and sunny summer and one that is either cool and wet, or unbearably hot and dry. As greenhouse gases continue to rise we know that there will be a general warming over Europe as a whole but these results, projected to the future, suggest that, on top of that warming, variations in summer temperature are also strongly controlled by where the storms happen to be tracking, and the position of the storm track seems to be random rather than forced, and is therefore unpredictable”.

Future changes in the position of the storm track could lead to strong regional contrasts in the amount of warming. Professor Danny McCarroll, also of Swansea University said: “We know that the climate of Europe is going to continue to warm, but the long-term perspective provided by this study suggests that, in any one place, we are unlikely to see a steady, unbroken rise in summer temperatures. We could see unpredictable shifts in the position of the summer storm track, leading to runs of unusually warm and dry or cool and wet summers and those shifts could persist for many years or even decades.”

The study, examined instrumental climate measurements, climate reconstructions from lake sediments and tree rings, and climate model simulations for the last thousand years. The results show that as the average summer temperature of Europe rises and falls through time, in response to factors such as greenhouses gases, energy from the sun and volcanic eruptions, the differences in temperature between northern and southern Europe do not change in parallel – instead they follow their own pattern and do not seem to be predictable.

The unpredictable component of the changes in summer temperature is linked to the position of the summer storm belt crossing the Atlantic, which determines where most of the summer rains will fall.

The study found:

When storm systems take a northerly path, summers are wetter and cooler in the north, and warmer and drier in the south.

A southerly track leads to hot dry summers in northwest Europe.

Over the last thousand years, the summer storm track seems to have wandered at random.

Unlike the average temperature across the continent, summer storm tracks do not seem to be controlled by known climate forcing factors.